Field of the Invention
The invention relates to a circuit configuration for monitoring a supply voltage of a processor unit being clocked by an oscillator and requiring a minimum supply voltage.
One such circuit configuration has been made by Intersil, for instance, in its ICL 7665 integrated circuit. Details of that circuit can be found, for instance, from the Intersil Component Data Catalog 1986, pp. 5-39 through 5-46. That circuit is essentially a comparator with low power consumption, which is highly advantageous for battery-operated applications. The comparator compares the supply voltage of a processor unit with a reference voltage and if the supply voltage drops below the reference voltage, it puts the microprocessor into a defined reset state. However, in certain operating situations the processor unit can enter into undefined states that lead to high quadrature-axis components of current. That unnecessarily discharges the battery and threatens the processor unit. One such problematic operating situation is, for instance, where an attempt is made to activate the processor unit, at an overly low supply voltage. Activating the processor unit at a supply voltage above its minimum supply voltage can also cause problems, if the supply voltage drops back below the minimum supply voltage as a result of the turn-on and the attendant higher power consumption. In battery,operated applications, a change of batteries, in which either charged or uncharged batteries are installed, can cause such undefined states as well. Finally, impermissible operating situations also occur if the supply voltage rises above the minimum supply voltage before the processor unit is operating properly, during the startup phase of the oscillator.